Image caption
If less than 70% of badgers are killed, the incidence of TB will not be reduced - and may even increase

Farming groups claim that the proposed badger culls will reduce the incidence of TB in cattle by 30% or more. Critics of the plan say that the impact on TB rates is so small that the scheme is not worthwhile; some even describe it as "crazy". Pallab Ghosh assesses the science behind the claims and counter claims.

An eight-year trial carried out in the 1990s showed that a sustained and coordinated culling of badgers can slightly slow down the rate of increase in cattle becoming infected with TB in the immediate area. It also showed that there was an increase in TB infections outside the cull area.

The so-called Krebs trial showed that there is a 16% reduction in the rate of increase for a 150 sq km area (60 sq miles) if more than 70% of badgers are killed in a series of culls held once a year for four years.

If less than 70% of badgers are killed, the incidence of TB will not be reduced and may even increase because of the greater movement of badgers caused by the culling. Badgers move in to an area because of the greater availability of food and habitats following culling.

Two pilot six-week culls have been given the go ahead by Natural England. These use a cheaper method of culling called "free shooting", which involves laying out bait and shooting badgers at night. The Krebs trail involved trapping badgers and shooting them in their traps cleanly and at close range during the day.

An independent group led by Professor Chris Wathes of the Royal Veterinary College will assess whether the new trials kill enough badgers (more than 70%) and whether they are killed humanely.

Effective and humane

Professor Wathes is expected to produce his report by the end of the year. If he and his team conclude that the culls are effective and humane, the Department for Environment, Food and Rural Affairs (Defra) will be able to issue licences for culls across the country. Each will be six-week culls held once a year for four years.

The scientist who designed the Krebs trial, Lord Krebs, has described the pilots as a "crazy scheme". One of his chief concerns is that Defra's current methods for assessing badger numbers for the pilots are extremely approximate, and unless they are improved it will be hard to assess whether more than 70% of badgers have been shot and therefore whether the pilots have succeeded or failed.

Cattle TB: Situation in 2012

About 26,000 cattle were slaughtered in 2011

23.6% of herds in the South West of England had an instance of TB during 2010

Defra's Food and Environment Research Agency is attempting to find better ways of assessing badger populations.

Opponents of culling argue that a 16% decrease in the rate of increase in infections gives a relatively small benefit and is not worth it, especially when the cost of licensing culls across the country and policing the anticipated protests are taken into account.

Instead, they say that it is better to tighten up measures to identify and cull infected cattle, build better fences to stop badgers coming into contact with cattle and to find ways of making vaccination of badgers and cattle more effective. The Welsh and Northern Irish Assemblies have opted for this approach (TB in cattle in Scotland is not a problem).

But vaccines and increased biosecurity are unlikely to make much impact in the short term. TB will probably continue to infect increasing numbers of cattle across England.

The National Farmers Union (NFU) and British Veterinary Association (BVA) say that something has to be done. Even if culling delivers only a modest benefit, it is better than nothing, they argue. Moreover, the farmers themselves believe that the benefit from culling to them is sufficient for them to be willing to pay for a large proportion of the costs.

The cost of cattle TB

TB in cattle is increasing slowly but surely, particularly in South West England. The number of cattle slaughtered in England in 2011 to control TB was 26,000. On average, the cost of a TB infection on a farm is £34,000. The government picks up the bulk of this cost, about £22,000, leaving the farmer to pick up the final £12,000.

Defra estimates that it will cost farmers and the government a total of £1bn over the next decade if no further action is taken.

Conflicting figures

Spokespeople for the NFU often say in interviews that continuous and systematic culling can reduce instances of TB by 30% or more. This is the figure for the reduction in the incidence in TB in the fifth year after the cull first began - the best possible outcome that can be used to back the case for culling. But after the cull has stopped, the reduction in the infection rate is rapidly eroded as infected badger numbers recover.

When challenged over the use of this figure, Philip Hudson, the NFU's head of food and farming agrees that the 16% figure for 150 sq km area is a more representative number.

Defra, Lord Krebs and other scientists also prefer the 16% figure, which is an average over nine years, because it gives an indication of the long-term benefit - rather than a short-term effect.

Perturbation effect

The process of culling causes a movement (perturbation) of badgers that increases the infection rate just outside of the culling zone. This is why the licences require a minimum of 150 sq km, which is the threshold for culling to be effective: the bigger the culling area the smaller the proportion of area outside.

Farmers have argued that because the pilot areas are 300 sq km (115 sq miles) - twice the size of the threshold - the reduction in the increase in infection rates will be higher than the 16% for 150 sq km. They are correct - but the extra benefit is modest at 19%.

Pro-cull voices argue that because the pilot areas have "hard boundaries", such as rivers or motorways, the badgers will not be able to move in, and so infections will be reduced still further.

The Krebs trial, however, used hard boundaries whenever possible, and so scientists who worked on that study doubt farmers will do any better than they did.

Alternatives to culling

Biosecurity measures have in the main failed to stop the year-on-year increase in cases.

There is not yet a licenced cattle vaccine and the badger vaccine is relatively expensive. In any case, reduction in TB rates would accrue gradually because badgers infected before the vaccination programme would remain in the countryside. But in a few years, vaccines could offer huge benefits.

Cattle vaccines cannot currently be used because it is impossible to tell the difference between a vaccinated animal and an infected animal, and so meat and dairy products from the animal cannot be sold.

In the medium term, it will be possible to develop a cattle vaccine that allows infected and vaccinated animals to be distinguished, but the UK would still have to persuade trading partners to accept vaccinated products. A badger vaccine has been produced, but it has to injected and is therefore of limited practical value. An oral vaccine for badgers that would be much more practicable is said to be a few years off.

Greater risks

There is concern though that this night-time "free shooting" method increases the risk of accidental injury, especially if protesters attempt to disrupt the culling.

With cage trapping, badgers were shot at very close range with very high accuracy. Shooting uncaged badgers raises the possibility of badgers being injured and suffering before they are killed.

Defra's expert scientific advisory group has said that because the culling method used in the new trial is different to the one used in the Krebs trial, the projected reductions in cattle TB may not be the same: they may be greater or they may be less.